Wireless communication systems include paging systems, trunk group communication systems, cordless telecommunication systems, and cellular mobile communication systems. Current important research topics in wireless communication systems focus on increasing system subscriber capacity and spectrum utilization rate, while reducing systems costs. State of the art wireless systems must provide a variety of features including voice communication, paging, message transmission, group dispatch communication, locating position features, and data communication.
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths and must make highly efficient use of the scarce bandwidth resource for providing quality service to a large population of users. Code Division Multiple Access (CDMA) protocol is commonly used for wireless communications systems for making efficiently use of limited bandwidths.
The use of wide-band multi-carrier RF technology and Time Division Duplex (TDD) technology in wireless communication systems provides a variety of advantages including increased spectrum utilization rates, low cost RF components, and simplified RF circuit structures. The use of wide-band multi-carrier RF and TDD technologies in wireless communication systems has also allowed for improved locating position features for determining the positions of subscriber terminals. Furthermore, wide-band multi-carrier RF and TDD technologies have allowed for efficient use of digital beam forming (DBF) based on a smart antenna for the purposes of reducing multi-path fading, raising coverage ranges, improving the accuracy of locating positions, and reducing the transmit power required for subscriber terminals.
The of multi-carrier RF transceiver systems reduces the complexity and costs of TDD base stations. Time division duplex (TDD) radio transceiver systems are used for providing duplex radio communications by alternately transmitting and receiving on a time division basis. In such systems, a transmitter and a receiver operate in time division duplex to transmit and receive respectively in different time slots. Therefore, a single antenna needs to be connected at any instant to either the transmitter or receiver but not to both simultaneously.
For wide-band multi-carrier TDD radio transceiver systems, it is important to optimize the linearity of multi-carrier RF circuits used therein. For example, a multi-carrier RF transceiver must be linear enough to ensure that the 3rd-order intermodulation distortion (IM3) of the transmitter amplifier is less than some value. For example, it is desirable that the intermodulation distortion of the transmitter amplifier have a value of -60 dBc while the transmitter provides an output power of +40 dBm. Conventional antenna switching circuits used in wide-band multi-carrier TDD radio transceiver systems do not meet this criteria. Therefore, the development of TDD wireless communications system has been limited.
Some antenna switching circuits have been designed for lower transmitting power wireless communication systems (e.g., CT2, PHS). However, these antenna switching circuits are not easily adapted to provide higher output power if it is desired.
FIG. 1 shows a schematic circuit diagram at 10 of a typical prior art full duplex transceiver having time division duplex (TDD) features. The transceiver 10 includes: a digital signal processor (DSP) 12 having a first port 14, and a second port 16; a transmitter subsystem 18 having a first port 20 coupled for communication with port 14 of the DSP, and a second port 22 for providing a transmission signal; a receiver subsystem 24 having a first port 26 coupled for communication with port 16 of the DSP, and a second port 28; and an antenna switching circuit 36 having a port 37 coupled to port 22 of the transmitter subsystem via a cable 35, and a port 38 coupled to provide a received RF signal to port 28 of the receiver subsystem 24 via a cable 39.
The antenna switching circuit 36 includes: an RF circulator 40 formed by a three-port transfer device having a first port 42, a second port 44 coupled to receive the transmission signal from port 22 of the transmitter subsystem, and a third port 46; a band pass filter 48 having a first port 50 coupled with the first port 42 of the RF circulator 40, and a second port 52 coupled with an antenna 54; a switch 56 having a first port 58 coupled with port 46 of the circulator, a second port 60, and a third port 62 coupled with port 28 of the receiver subsystem 24 via port 38 of the antenna switching circuit; and a load impedance 64 having an impedance value RL, and having a first terminal connected to the second port 60 of the circulator, and a second terminal connected to ground.
The transmitter subsystem 18 includes: a transmitter signal processing unit 70 coupled for communication with port 14 of the DSP via port 20 of the transmitter subsystem; a modulator unit 72 having a first port coupled to receive a base band signal from unit 70 via a path 74; a transmitter 76 having a first port coupled for communication with modulator 72 having a first port coupled to receive a base band signal from unit 70 via a path 78; and a power amplifier 30 having an input 32 coupled to receive a signal from transmitter 76, and an output 34 providing the transmission signal at port 22 of the transmitter subsystem. The receiver subsystem 24 includes: a receiver signal processing unit 80 coupled for communication with port 16 of the DSP via port 26 of the receiver subsystem; a demodulator unit 82 coupled to unit 80 via a path 84; and a receiver 86 having a first port coupled for communication with demodulator 82 via a path 88, and a second port coupled for communication with the third port 62 of switch 56 of the antenna switching circuit via the cable 39.
The circulator 40 facilitates signal transfer in an upstream direction, and minimizes signal transfer in a downstream direction. For signals propagating in the downstream direction (from port 44 to port 42, from port 42 to port 46, and from port 46 to port 44), insertion loss is approximately 0.7 dB. For signals propagating in the upstream direction (from port 42 to port 44, from port 44 to port 46, and from port 46 to port 42), insertion loss is approximately 30 to 40 dB. The band pass filter 48 attenuates unnecessary radio waves in both the transmitting mode and the receiving mode. In a TDD mode, the transmitting frequency is substantially equal to the receiving frequency.
In a transmit mode of operation of the depicted transceiver 10, switch 56 of the antenna switching circuit is controlled so that its first port 58 is connected to its second port 60. In a receive mode, switch 56 of the antenna switching circuit is controlled so that its first port 58 is connected to its third port 62.
In the transmit mode, a base-band signal is generated by unit 70, modulated by modulator 72, transmitted by the transmitter 76, and amplified by amplifier 30 to generate the transmission signal at port 22 of the transmitter subsystem. The circulator 40 of the antenna switching circuit is operative to circulate the transmission signal received at its second port 44 to its first port 42 which is connected to the antenna 54 via the band pass filter 48. The generation of the transmission signal by the transmitter subsystem 18 and the switched path of the transmission signal by the switching circuit 36 operating in the transmit mode is illustrated by a transmission path 90.
A reflection problem arises in the transmit mode if the impedances of antenna 54 and filter 48 are not properly matched to the impedance of the first port 42 of the circulator. Such an impedance mismatch may arise due to antenna match errors, filter match errors, or variations in environmental conditions. In the event of such an impedance mismatch, RF power is reflected from the antenna 54 back to port 42 of the circulator. However, the circulator is operative to circulate this reflected RF power from port 42 to port 46 which is connected to port 58 of switch 56. As mentioned above, in the transmit mode, switch 56 is set so that port 58 is coupled to port 60, and therefore a transmission reflection path 91 is created so that any RF power reflected from the antenna and band pass filter is circulated from port 42 to port 46, transferred by switch 56 from port 58 to port 60, and absorbed by the load impedance 64. Note that any reflected RF power received at port 42 is substantially isolated from the transmitter subsystem 18 by the circulator 40 because port 44, which is connected to the transmitter subsystem, is upstream from and therefore substantially isolated from port 42 as explained above.
In the receive mode of operation, wherein port 58 of switch 56 is connected to port 62, a receive path 92 is created so that an incoming signal received at antenna 54 is provided via the switching circuit 36 to port 28 of the receiver subsystem 24. The incoming signal received at the antenna propagates along the receive path 92 which extends from antenna 54 to receiver signal processing unit 80, and which traverses filter 48, ports 42 and 46 of circulator 40, ports 58 and 62 of switch 56, receiver 86, and demodulator 82.
A reflection problem arises in the transceiver 10 during operation in the receive mode if the input impedance of the receiver 86 and the impedance of cable 39 are not properly matched. In this case, a portion of the incoming signal provided from the antenna to the receiver subsystem 24 is reflected from the input of the receiver 86 back to port 46 of circulator 40 via switch 56. The circulator is operative to circulate the reflected portion of the incoming signal from port 46 to port 44 which is connected to the output of the power amplifier 30 via cable 35. Because the power amplifier 30 is OFF during operation in the receive mode, and because the output impedance at the output of the power amplifier 30 can not be matched with the impedance of the cable 35 while the amplifier 30 is OFF, a second portion of the incoming signal is reflected by amplifier 30 back toward port 44 of the circulator. The circulator is operative to circulate the second reflected portion of the incoming signal from port 44 to port 42 which is connected to the antenna via the band pass filter.
Therefore, incoming signals provided to the receiver subsystem 24 are reflected along a receive signal reflection path 93 which includes: a first portion extending from the input of the receiver 86 to the output of the power amplifier 30 via switch 56 and circulator 40; and a second portion extending from the output of amplifier 30 to the antenna via ports 44 and 42 of the circulator 40, and the band pass filter 48. Note that if another impedance mismatch is also present at the antenna 54, a third portion of the incoming signal is reflected yet again from the antenna 54.
With reference to FIG. 2, in order to prevent the above described reflection problems arising in the transceiver 10 (FIG. 1) during operation in the receive mode, a second circulator 98 is inserted into the antenna switching circuit to provide isolation between the switch 56 of the antenna switching circuit and the receiver subsystem 24 of the transceiver 10 (FIG. 1). As shown in FIG. 2, an improved prior art transceiver circuit 96 comprises the DSP 12, transmitter subsystem 18, receiver subsystem 24, and an improved antenna switching circuit 97 which includes all of the components of the antenna switching circuit 36 (FIG. 1) in addition to: the second circulator 98 which has a first port 99 connected to port 62 of switch 56, a second port 100, and a third port 101 connected to the input of the receiver 86 via port 28 of the receiver subsystem; and a load impedance 102 having a resistance of 50 Ohms, and having a first terminal connected to port 100 of the second circulator, and a second terminal connected to ground.
Switch 56 of the antenna switching circuit 97 is controlled in the same manner as described above in the switching circuit 36 of the transceiver 10 (FIG. 1). During operation in the receive mode, any RF energy reflected by the receiver 86 is reflected back to port 101 of circulator 98, circulated from port 101 to port 100, and provided to the load impedance 102 which absorbs the reflected RF energy.
The insertion of the second circulator 98 into the transceiver 10 (FIG. 1) to form the transceiver 96 provides for a receive path 104 having an insertion loss which is approximately 0.7 dB larger than the insertion loss of the receive path 92 of transceiver 10 (FIG. 1). Therefore, the receiver sensitivity of the transceiver 96 is decreased by 0.7 dB relative to the transceiver 10 (FIG. 1) as a result of the addition of the second circulator 98 to the circuit.
Assume that the insertion loss of the band pass filter 48 is about 1.3 dB, the insertion loss of the electronic switch 56 is about 0.7 dB (e.g., if switch 56 is implemented by a Stanford Microdevices switch, model SSW-224), and the insertion loss of each of the circulators 92 is approximately 0.7 dB. In this case, the insertion loss of the transmitting path 90 (FIG. 1) is approximately 2.0 dB, and the insertion loss of the receiver path 104 (FIG. 2) is approximately 3.4 dB.
Assume that the required output power of amplifier 30 is 10 Watts, or 40 dBm. Due to the insertion loss of the transmitting path 90, the output power delivered to the antenna is reduced to approximately 6.3 Watts. Therefore, the first circulator 40 and the band pass filter 48 consume 37 percent of the power output by the amplifier 30. If 10 W is required at the antenna output port, then the required output power of the power amplifier 30 is 15.8 W. Such a power requirement increases the difficulty of designing a transceiver. It is even more difficult to implement a linear multi-carrier transceiver wherein the IM3 is less than -60 dBc. In such a case, the efficiency of the amplifier 30 is only approximately 5 to 10 percent.
Still referring to FIG. 2, note that two cables 35 and 39 are required to connect port 22 of the transmitter subsystem 18 and port 28 of the receiver subsystem 24 to the antenna 54 via the antenna switching circuit 97. In order to optimize system performance, high quality cables must be used to implement the cables 35 and 39, and such high quality cables are costly. Therefore, it would be advantageous if only one cable were required for establishing the connection between the transmitter subsystem and receiver subsystem and the antenna 54 via an antenna switching circuit. The savings in cost and reduced complexity as a result of requiring only one cable for this connection would be particularly advantageous in a wireless communication base station system which typically includes a plurality of antennas configured in an antenna array.